This invention relates to a biaxially stretched heat-shrinkable laminate film which possesses good heat-shrinkability, excellent melt hole resistance (ability to resist formation of melt holes) and heat-resistant sealing property (the heat resistance of a seal portion), and outstanding cold resistance, serves satisfactorily as a packaging material suitable for wrapping food, comprises therein as an oxygen barrier layer a layer of a mixture of vinylidene chloride resin with a low melting point polyamide resin having a crystalline melting point of not higher than 210.degree. C., has the mixed resin cross-linked by applying electron beam to the laminated film at the range of 1 to 12 megarads, exhibits a heat shrinkage percentage of not less than 15% at 90.degree. C., and excels in the gas-barrier property.
Generally shrinkable packages are most convenient for the purpose of wrapping such fatty foodstuffs as raw meat, processed meat, and cheese which are ununiform and irregular in shape. Since these food are required to be stored for a long time, they require not only an excellent gas barrier property [less than 200 (cc/m.sup.2 .multidot.day.multidot.atom) under the conditions 30.degree. C. and 100% RH] but also highly satisfactory functions such as heat sealability, cold resistance, melt hole resistance, and heat-resistance sealing property. Moreover, the transparency of the packaging material after shrinkage constitutes one of the important factors which affects the appearance of the packaged product. When a fatty foodstuff is packaged and pasteurized, it is often observed that the film softened with fat and heat is stretched to become thin and thus break (melt holes) and the stress of heat shrinkage generated during the course of pasteurization inflicts breakage along or near the sealed edge of the package. In the circumstances, the industry has required for a heat-shrinkable film having gas barrier property, melt hole resistance, heat-resistant sealing property, cold resistance which excludes the possibility of occurring pin holes and other similar flaws during a distribution in a low temperature, and retaining excellent transparency even after shrinkage.
The vinylidene chloride resin (hereinafter referred to briefly as "PVDC") film are used extensively because it excels in the first place in shrinkability and equally in gas barrier property, oil resistance, clipping property, and other qualities.
The conventional PVDC film contains 6 to 10% by weight of such additives as plasticizer and stabilizer for the impartation to itself of ample cold resistance, flexibility indispensable to the enhancement of clipping property, and highly satisfactory film-forming property. With foodstuffs of some sorts, these additives have the possibility of migration into the foodstuff and bring about effects undesirable sanitarily. Where the film is subjected to severe conditions such as are encountered in the wrapping of heavy articles, it has the possibility of betraying its deficiency in strength and particularly in cold resistance. Thus, the desirability of supplying the industry with an excellent packaging material free from such drawbacks has been finding enthusiastic recognition.
For the purpose of eliminating these drawbacks, there has been proposed a three-layer film which is produced by coextruding an intermediate layer of PVDC containing such additives as plasticizer and stabilizer in very small or virtually negligible amounts and a pair of outer layers of ethylene-vinyl acetate copolymer (EVA) excelling in cold resistance and adhesion to the PVDC layer and which, consequently, overcomes the hygienic problem of the PVDC film and enjoys improved cold resistance (Canadian Patent No. 982,923).
Further, a laminate which comprises sequentially (1) a first layer containing an organic polymer (2) a gas barrier layer based on PVDC and allowed to endow the laminate with an oxygen permeation rate of not more than 70 cc/m.sup.2 /day/atm (as measured under the conditions of 22.8.degree. C. and 0% RH in accordance with ASTM 1434), and (3) a layer containing an organic polymer capable of withstanding severe handling and owns flexibility suitable for a heat-shrinkable package and which is characterized by the fact that the layer 1 contains an oriented copolymer of ethylene and vinyl acetate containing 5 to 20% by weight of a unit originating in vinyl acetate and the copolymer is cross-linked by exposure to radiation, the gas barrier layer 2 contains 70 to 85% by weight of a unit originating in vinylidene chloride and 30 to 15% by weight of a unit originating in vinyl chloride, and the layer 3 contains (i) a copolymer of ethylene and vinyl acetate containing 5 to 20% by weight of a unit originating in vinyl acetate or (ii) a blend of isotactic polypropylene, atactic polypropylene, and polybutene-1 (Japanese Patent Publication No. 43024/1983, U.S. Pat. No. 3,741,253); a laminate which comprises at least three layers, including a gas barrier resin layer, a layer of a different thermoplastic resin, and an adhesive layer interposed therebetween and which is characterized by the fact that the adhesive layer is formed of a composition consisting of 100 parts by weight of a melt-extrudable adhesive resin and 0.1 to 50 parts by weight of a radiation-sensitive compound and is cross-linked by exposure to radiation (Japanese Patent Laid-open No. 11,342/1985, U.S. Pat. No. 4,699,846); a stretched film laminate which comprises (1) a substratal film layer containing an alpha-monoolefin polymer cross-linked by exposure to radiation and (2) a film layer containing a polymer cross-linkable by exposure to radiation and which, by exposure of itself to radiation, causes the polymer of the film (2) to be cross-linked and the polymer of the substratal film (1) layer further cross-linked (Japanese Patent Publication No. 47, 859/1986); a heat-shrinkable biaxially stretched multilayer film which includes a gas barrier layer containing a vinylidene chloride-methyl acrylate copolymer and which is exposed to radiation of a dosage level approximately in the range of 1 to 5 megarads, and suitable for wrapping slices of primal, subprimal meats (Japanese Patent Laid-open No. 3948/1987); and a molecular oriented multilayer polymer film which comprises a first layer and a second layer each formed of a composition containing an ethylene-vinyl acetate copolymer in a predominant proportion and a third layer formed of a composition of PVDC and interposed between the first and second layers and which has the first, second, and third layers joined by cross-linking bondage due to exposure to electron beam in a dosage of not less than 1.5 megarads (Japanese Patent Laid-open No. 23,752/1987) have been proposed to the art.
Besides, laminated films incorporating as outer layer therein a polyolefin layer or an EVA layer cross-linked by exposure to radiation have been proposed by Japanese Patent Laid-open No. 3,456/1972, Japanese Patent Publication No. 20,549/1979, U.S. Pat. No. 4,044,187, U.S. Pat. No. 4,064,296, U.S. Pat. No. 4,352,844, U.S. Pat. No. 4,501,780, Japanese Patent Publication No. 5553/1968, Japanese Patent Publication No. 20599/1971, Japanese Patent Publication No. 44,019/1976 and, Japanese Patent Publication No. 44,020/1976, British Patent No. 2,040,804, U.S. Pat. No. 4,391,862, U.S. Pat. No. 4,448,792, U.S. Pat. No. 4,514,465, and U.S. Pat. No. 4,551,380, etc.
Though the conventional laminated films such as, for example, the film of the multilayer construction of EVA/PVDC/EVA possess heat-sealability, good cold resistance and excellent gas barrier property and make up for the drawbacks suffered by the film formed solely of PVDC, they are deficient in melt hole resistance and heat-resistant sealing property.
For the purpose of improving these conventional laminate films in heat resistance, the cross-linking technique relying on the exposure to radiation as disclosed in the aforementioned patent publications has been introduced to the art. The crystallized PVDC is a copolymer of the type which, on exposure to electron beam, undergoes degradation attended by molecular scission. When a laminate incorporating therein a layer of crystallized PVDC is exposed to electron beam, therefore, it suffers impairment caused by the .sub.poor cold resistance.
For the purpose of imparting heat shrinkability to the laminate having a pair of polyolefin layers opposed to each other across an intervening PVDC layer, the practice of stretching the laminate at a temperature not more than 40.degree. C. lower than the crystalline melting point of the polyolefin is generally employed. This stretching, therefore, is incapable of imparting a fully satisfactory effect of orientation to the PVDC layer of the laminate. Since the PVDC layer is consequently deficient in heat shrinkage percentage, it is left behind in the behavior of shrinkage when the laminate is subjected to heat shrinkage. As the result, the PVDC layer bends limply and tends to impair seriously the transparency of the laminate after shrinkage.
In the field of food packaging, therefore, an earnest demand is expressed for a heat-shrinkable film which possesses not only satisfactory gas barrier property and high cold resistance but also outstanding melt hole resistance and heat-resistant sealing property and, at the same time, retains excellent transparency even after shrinkage.
After a diligent study aimed at overcoming the aforementioned drawbacks suffered by the conventional techniques, the present inventors have found that when a laminate incorporating therein as a gas barrier layer a layer of a mixture of 5 to 50% by weight of PVDC and 95 to 50% by weight of a low melting point polyamide resin is exposed to electron beam at a dosage in the range of 1 to 12 megarads, since the mixed resin is cross-linked, the layer of the mixed resin can be endowed with heat-shrinkability by a subsequent step of stretching and the laminate can be vested with low-temperature impact strength and gas barrier property. It has been further found that, in this case, it is desirable from the standpoint of protecting the laminate against coloration after exposure to the electron beam and, at the same time, enhancing the degree of cross-linking that the PVDC should incorporate therein a cross-linking agent and should be then blended with the low melting point polyamide resin. The present invention has been accomplished on the basis of this knowledge.